Related-Party Transactions - Summary of Management Fees, Incentive Fees and Associated Credits and Administration Fees (Detail) - USD ($) $ in Thousands  | 
12 Months Ended | |
|---|---|---|
Dec. 31, 2020  | 
Dec. 31, 2019  | 
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| Related Party Transactions [Abstract] | ||
| Base management fee | $ 4,289 | $ 3,623 | 
| Incentive fee | 3,038 | 847 | 
| Capital gains fee | 0 | 0 | 
| Credits from non-contractual, unconditional, and irrevocable waiver granted by Adviser's board of directors | 0 | (1,543) | 
| Total fees to our Adviser, net | 7,327 | 2,927 | 
| Administration fee | 1,448 | 1,207 | 
| Selling Commissions and Dealer-Manager Fees | 4,711 | 7,645 | 
| Financing fees | 262 | 235 | 
| Total fees to Gladstone Securities | $ 4,973 | $ 7,880 | 
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- Definition Administration fee. No definition available. 
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- Definition Capital Gains Fee No definition available. 
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- Definition Credits to fees from adviser. No definition available. 
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- Definition Financing Fee No definition available. 
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- Definition Management Fees and Financing Fees No definition available. 
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- Definition Related party transaction net incentive fees paid and credits received. No definition available. 
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- Definition Selling Commissions and Dealer-Manager Fees No definition available. 
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- Definition Amount of expense for incentive rights held by the managing member or general partner, of limited liability company (LLC) or limited partnership (LP). Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Amount of expenses related to the managing member or general partner for management of the day-to-day business functions of the limited liability company (LLC) or limited partnership (LP). Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- References No definition available. 
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